There is growing market demand for LCD's including demand for their use in outdoor environments. The use of LCD's in outdoor environments entails exposing the displays to temperatures that may be significantly above or below optimal for LCD performance.
Liquid crystals are characterized by their ability to change their optical properties in response to applied electromagnetic fields. This has made them ideal for displaying information that changes periodically, such as in liquid crystal displays. This ability is affected by the temperature of the liquid crystals, which is in turn dependant upon a number of factors, such as air temperature, absorbed radiation from the sun, and heat generated by electronic equipment in the vicinity of the display etc. The effects of varying LCD temperature is especially pronounced when the optical state of the liquid crystals is determined by a low-voltage multiplexing technique, since this puts high requirements on the flexibility of the crystals.
A LCD's performance is temperature-dependant, and in particular, performs poorly at low temperatures. In an effort to counter this; integral heaters have been provided in LCD devices to raise the temperature of the LCD to achieve satisfactory performance. The heaters generally include LCD temperature sensors often connected to a microprocessor which controls a switch to modulate the heating implement and maintain the LCD at a satisfactory operating temperature.
Liquid crystal displays do not operate well at low temperatures. At temperatures of approximately −20 degrees C. and below, the LCD fluid becomes too viscous to respond to an applied electric potential within an acceptable time. Because it is the fluid in the LCD and not the remainder of the device that must be heated during low temperature operation, LCD's can be procured with integral heater elements.
In the vast majority of displays the temperature sensor is located on one side of the LCD, most likely outside of the viewable area of the LCD. In this arrangement the sensor detects temperatures that while they are similar to that of the liquid crystals, there is room for improvement. While this phenomenon is most likely due to thermal coupling with metal in the display assembly, this offset appears to vary from finished assembly to finished assembly, and thus is difficult to account for in manufacture.
If the temperature sensor does not accurately reflect the temperature of the liquid crystal, the resulting heating of the display will be suboptimal. Slow heating or failure to reach a threshold temperature can cause the LCD to operate in an unacceptable fashion, as the liquid crystal fluid will be too viscous to respond suitably to applied currents. Conversely, if the heating progresses beyond desired temperatures the operation and lifetime of the display are compromised.
It is therefore desirable to determine as close as possible the actual temperature of the liquid crystal matrix, if the temperature is not determined accurately, the temperature compensation system (for example a switch to turn off an integral metal heater layer) will not function optimally.